Electric motors are used in a variety of electrical equipment. For example, linear electric motors produce electrical power propelling an armature in one dimension. Wafer stages utilize linear electric motors to position a wafer during photolithography and other semiconductor processing.
A typical one-dimensional linear electric motor has a magnet track with pairs of opposing magnets facing each other. (Copending U.S. Ser. No. 09/059,056, entitled "Linear Motor Having Polygonal-Shaped Coil Unit" filed on Apr. 10, 1998, by Hazelton et al. (Attorney Docket No. M-5107US) discusses one-dimensional linear electric motors and is incorporated herein by reference in its entirety.) Within spaces between the pairs of opposing magnets, an armature moves. The armature has windings of a conductor which are connected to an electrical current. When the electrical current is turned on, the electric current interacts with the magnetic fields of the magnet pairs to exert force on the armature, causing the armature to move. When the armature is attached to a wafer stage, the wafer stage experiences the same force as and moves in concert with the armature.
In a multiphase motor, the armature has various windings grouped into phases. The phase groups are selectively pulsed with electric current to create a more efficient motor. As the armature moves within the magnet track as a first group of coils is pulsed, the first group moves out of its optimal position between the pairs of magnets. Then, it becomes more efficient to pulse a second group of windings. More phase groups are theoretically more efficient since a more even application of force and utilization of power input is maintained. However, each additional phase group complicates a timing of the pulses to the various phase groups. Presently, three-phase motors and armatures have gained favor in balancing these considerations.
Linear two-dimensional motors are also used in manufacturing. (U.S. Pat. No. 4,654,571, entitled "Single Plane Orthogonally Moveable Drive System," issued to Hinds on Mar. 31, 1987, U.S. Pat. No. 4,535,278, entitled "Two-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatus," issued to Asakawa on Aug. 13, 1985 and copending U.S. Ser. No. 09/192,813, entitled "Electric Motors and Positioning Devices Having Moving Magnet Arrays and Six Degrees of Freedom" filed on Nov. 16, 1998, by Hazelton et al. discuss two-dimensional linear electric motors and are incorporated herein by reference in their entireties.) The motors are two-dimensional in that they have two-dimensional arrays of magnets and armatures instead of magnet tracks and one-dimensional armatures. However, the magnet arrays and two-dimensional armatures may move with respect to each other in more than two dimensions depending upon the design. Conventional two-dimensional linear motors typically have an array of magnets and an armature having one or more coils on one side of the array of magnets.
When attached to part of a two-dimensional linear motor, a platform can be moved in two or more dimensions by the motor. For example, a wafer stage in semiconductor processing equipment may be attached to an armature or magnet array of a two-dimensional motor, and the two-dimensional motor would control positioning of the wafer stage.
Electric motors are also used to provide motion of platforms or stages in six degrees of freedom, three translational and three rotational. (Richard P. Feynman, T. Leighton, and M. Sands, The Feynman Lectures on Physics, Addison-Wesley, 1962, discusses translational and rotational motion and degrees of freedom and is incorporated by reference herein in its entirety.) Unfortunately, many designs obtain six degrees of freedom by essentially stacking one dimensional motors which move only in two dimensions within a plane. (U.S. Pat. No. 5,623,853, entitled "Precision Motion Stage with Single Guide Beam and Follower Stage" issued to Novak et al. on Apr. 29, 1997, discusses examples of such stacked arrangements and is incorporated herein by reference in its entirety.) For example, a stage may be propelled back and forth in one dimension under the control of linear electric motors. The linear electric motors are part of a holder which holds the stage. In turn, a second holder holds that entire holder and stage arrangement via joint connections and propels it back and forth in a second dimension by another set of linear electric motors. Additional degrees of motion may be provided by additional electric motors which are attached to these holders.
These types of stacked arrangements have a few drawbacks. Each additional holder enabling more degrees of freedom also adds mass requiring additional power for the electric motors to move the stage. Also, the complicated joint connections degrade accuracy of positioning of the stage and build-in resonant frequencies.
An alternative design is discussed in U.S. Pat. No. 5,196,745, "Magnet Positioning Device", issued to Trumper on Mar. 23, 1993 ("Trumper") which is incorporated by reference herein in its entirety. Trumper describes a planar motor comprising several linear motors formed between two planes. On the first plane, halves of conventional one dimensional magnet tracks used in one dimensional electric motors are placed about the plane. On the second plane, coils similar to those of one dimensional linear electric motors, are placed on areas on the second plane corresponding with the halves of the magnet track on the first plane. By commutating the coils on the second plane, Trumper achieves six degrees of freedom. However, since the Trumper design is based on a one dimensional linear electric motor design, the coils and the corresponding halves of magnet tracks must remain in close proximity to each other, thereby limiting the range of motion.
What is needed is a two dimensional electric motor that obviates the need for stacked stages and has a broader range of motion than that of the Trumper design. By eliminating the stacked stages, motor power requirements for motion would be lower. Additionally, fewer moving parts results in higher resonant frequencies and greater stability of motion. A wider range of motion permits greater choice of positions.